The present invention relates to a disk production apparatus and a disk produced by the disk production apparatus. Particularly, this invention relates to a disk production apparatus achieving high concentricity (or low eccentricity) between the center hole of a molded transparent substrate and tracks on a data-recorded zone formed on the substrate and a disk on which the disk center hole and tracks on a data-recorded zone have high concentricity.
Optical disks have been widely used thanks to large storage capacity at high density and recordability and reproducibility without being contact with a recording/reproducing optical head.
There are several types of optical disk according to storage density depending on laser wave lengths for optical heads.
Compact disks (CD) are subjected to recording/reproduction with a track pitch of 1.6 μm and the minimum pit length of 0.9 μm at a wavelength of 780 nm for laser beams.
Digital versatile disks (DVD) are subjected to recording/reproduction with a track pitch of 0.74 μm and the minimum pit length of 0.4 μm at a wavelength of 635 nm for laser beams.
Optical disks for digital video recorders (DVR) are subjected to recording/reproduction with a track pitch of 0.32 μm and the minimum pit length of 0.16 μm at a wavelength of 400 nm for laser beams.
There are several types of recording format for optical-disk substrates (transparent resin substrates) according to track pitches, pit lengths and modulation techniques.
Nevertheless, it is the same for these optical disks that data are recorded thereon in the form of spiral uneven configurations such as groves or pits.
These data groves or pits are formed in the process called mastering. Produced at the last stage of mastering is a disk-like stamper having spiral uneven configurations of data groves or pits formed on a metallic plate thereon.
The disk-like stamper is set on an optical-disk metal mold of an optical-disk production apparatus (injection molding machine). Molten resin is shot into the metal mold and then cooled to produce optical-disk substrates.
The stamper has a center hole through which it can be held in the metal mold according to the size of the metal mold. In detail, the center hole is punched through the center of a spiral of data grooves or pits. The outer edge of the stamper is cut away into, usually, a round shape to fit the mold outer shape, thus the disk-like stamper being produced.
The disk-like stamper is then set in an optical-disk molding metal mold. The metal mold is then attached on the injection molding machine. Molten plastic material such as polycarbonate resin is shot into the metal mold and then cooled, thus an optical-disk substrate being produced. A center hole is punched through the optical-disk substrate with a center-hole punching tool such as a punch and a die.
The produced optical-disk substrate has data groves or pits formed thereon that have been transferred from the stamper. Formed on the substrate surface having the data groves or pits is a recording layer having reflecting and recording functions. Laminated on the recording layer are a protective layer, a dummy layer, a cover layer, etc., according to needs, thus an optical disk being produced.
Data is recorded on or reproduced from an optical disk while the disk is rotating about its center hole set at a center pin provided on a turn table of a recording/reproduction apparatus.
Recording/reproduction is performed while an optical head is tracking the data grooves or pits formed on the optical disk. Hence, the narrower the track pitch from CDs to DVDs and to DVRs, the higher the accuracy of tracking being required.
One requirement is thus the narrower the track pitch, the smaller the radial runout for the train of data grooves or pits per one rotation of an optical disk, which may otherwise cause inaccurate tracking of data grooves or pits.
Smaller radial runout is achieved with smaller mismatch (disk eccentricity) between the center of spiral data grooves or pits and the center hole of an optical disk.
Smaller disk eccentricity is achieved with higher accuracy of a punching technique for the stamper center hole and also higher accuracy of attaching the stamper on an injection molding machine.
It is, however, practically difficult to punch the stamper center hole so that it can meet the center of spiral data groove (pit) train on the stamper, resulting in low concentricity therebetween.
Production of a large number of stampers could offer several stampers of high concentricity (or low eccentricity), which inevitably driving up production costs.
High concentricity could be achieved with high accuracy of a stamper-attaching tool such as a stamper-inner-hole damper (or retainer). The retainer is inserted into the center hole of a stamper. It is further inserted into a particular section of a metal mold to lower the disk eccentricity.
This provides a small gap between the center hole of the stamper and the outer edge of the retainer, to enhance mechanical accuracy in attaching the stamper on the metal mold.
In addition, a bushing attached in the metal mold is inserted into an inner hole of the retainer so that the retainer can be attached into the metal mold. The gap between the inner hole of the retainer and the outer edge of the bushing is also made small.
This technique offers high concentricity at first, however, lowers the concentricity gradually as the retainer is worn. Abrasion can be checked by a particular instrument. The introduction of such instrument in optical-disk production however drives up production costs.
The center holes of stampers could be different in diameter from one another due to low accuracy of a stamper-hole punching machine, aging of the machine, inappropriate temperatures at production, etc. This requires a large number of retainers of outer diameters slightly different from one another, which leads to complicated parts managements and high costs.